Add changelog to PR template
Add changelog to PR template

Summary:
If anything will help me remember.
Closes https://github.com/Instagram/IGListKit/pull/359

Differential Revision: D4365179

Pulled By: jessesquires

fbshipit-source-id: e002bc6bc31f8d2af59a5d5303a2c5cb09d780b6

Sherlouk/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,dshahidehpour/IGListKit,charles0126dev/IGListKit,Instagram/IGListKit,charles0126dev/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,dshahidehpour/IGListKit,Instagram/IGListKit,Sherlouk/IGListKit,Instagram/IGListKit,Sherlouk/IGListKit,Instagram/IGListKit,charles0126dev/IGListKit,dshahidehpour/IGListKit